1. Field of the Invention
The present invention relates to a method of forming a suspended structure, and particularly, to a method utilizing a patterned photoresist as a sacrificial layer and subsequently etching a substrate for suspending a structure layer to form a suspended structure.
2. Description of the Prior Art
In the MEMS field, three major processing technologies are Si micromachining technology, LIGA processing technology, and micromechanical machining technology. Since Si micromachining technology can be incorporated into the IC manufacturing process, it has become the most popular processing technology for MEMS device fabrication.
A conventional method of manufacturing a suspended structure utilizing Si micromachining technology is illustrated with reference to FIGS. 1-3. As shown in FIG. 1, a silicon substrate 10 having a patterned structural layer 12 on a surface thereof is provided. Referring to FIG. 2, a photoresist layer is formed on the structural layer 12 and the silicon substrate 10. A lithography process is performed to define a pattern and thereby a first patterned photoresist 16 having an opening 14 is formed. As shown in FIG. 3, a wet etching process is performed with KOH etching solution to etch the silicon substrate 10 through the opening 14 to form a notch 18. Afterwards, the first patterned photoresist 16 is removed and thereby a suspended structure 20 is formed.
However, the conventional method of manufacturing a suspended structure has several disadvantages. Firstly, materials of the structural layer and the etching solution are confined within certain limits. For example, the KOH etching solution may corrode an aluminum structure layer. Using KOH etching solution must therefore be avoided when the structural layer comprises aluminum. In addition, the material of the etching mask will be confined within certain limits depending on the composition of the etching solution. Only a few materials, such as LPCVD Si3N4 or low stress Si3N4 for example, can serve as an etching mask when the KOH etching liquid is used. Secondly, the conventional method removes a part of the silicon substrate and forms a notch to keep the structural layer in suspension. The suspended structure may collapse due to surface tension of the etching solution. This will result in a broken suspended structure or a part of the suspended structure sticking to the silicon substrate. Finally, most of the conventional suspended structures are flat structures but not 3D structures. It is very difficult for conventional methods to design and manufacture a suspended structure having a bend of a predetermined angle by adjusting the process parameters. Thus, the products of the suspended structure manufactured by conventional methods can only be applied in a limited number of application fields.